This invention relates to a method and filter for separating chrominance and luminance signals from a composite video signal, such as the signal specified by the National Television System Committee (NTSC), in which the chrominance signal is superimposed on a high-frequency region of the luminance signal.
In recent years chrominance-luminance separation has been markedly improved by the use of multi-dimensional digital filtering. One conventional system, for example, employs a two-dimensional comb filter for filtering spatial frequencies on the horizontal and vertical axes, and a frame comb filter for filtering the signal on the time axis. The outputs of these filters are mixed in proportion to the degree of motion in the picture, so that increasing motion leads to increasing use of the output of the two-dimensional comb filter.
By exploiting the frame-to-frame phase reversal of the color subcarrier signal, in a still picture the frame comb filter achieves complete chrominance-luminance separation. If the picture is moving, however, the frame comb filter creates unwanted artifacts such as double images. The two-dimensional spatial comb filter avoids these artifacts but produces others, such as cross color (luminance crosstalk into the chrominance signal) and dot crawl, which can be particularly noticeable at the edges of objects. Overall, the frame comb filter works better for stationary parts of the picture, while the two-dimensional spatial comb filter works better for moving parts, and this is the reason for selecting the outputs of the two filters in proportion to motion.
To detect the degree of motion, the conventional system compares the video signal in different frames. More specifically, a motion detector takes the difference between the signal in two consecutive frames, passes this difference through a low-pass filter to remove high-frequency components due to the above-mentioned color phase reversal, and takes the absolute value of the result. As a result, the motion in the low-frequency luminance signals is detected. To detect chrominance motion and high-frequency luminance motion, the conventional motion detector also takes the absolute value of the difference between the signal input in one frame and the signal input two frame previously. The motion can be detected by selecting the larger of the two absolute values.
One problem with this conventional system is that the low-pass filter in the motion detector fails to reject high-frequency components of the chrominance signals having a horizontal frequency distribution extending to a large extent, in the low-frequency region of the luminance signal, causing such low-frequency components to be mistakenly interpreted as motion.
Another problem is that, since the low-pass filter rejects all high-frequency components, the conventional motion detector fails to detect motion restricted to high-frequency components of the luminance signal over a one-frame interval.
A more basic problem is that the conventional system produces a very clean image, with complete chrominance-luminance separation, in stationary parts of the picture, but allows considerable dot crawl and cross color in moving parts. The striking difference in quality between different parts of the same picture creates an odd impression.